pub fn string_study() {
    let str = "123";

    let mut  s3 = String::new();
    s3.push('a');
    s3.push('b');
    s3.push('c');

    let s4 = s3.to_string() + "456";

    println!("str: {}", str);
    println!("s3: {}", s3);
    println!("s4: {}", s4);

    for key in s4.split("b") {
        println!("key: {}", key);

    }
}